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Aromaticity

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Aromatic compounds are unsaturated cyclic molecules that possess additional stability as a result of the arrangement of $\pi$ electrons associated with the unsaturation of the ring system. Aromatic compounds are also known as arenes they can be carboxylic indicating that the ring skeleton contains only carbon atoms or hetero cyclic with at least one atom other than carbon in the ring.


Aromaticity is usually described in molecular orbital terminology. Aromaticity is now generally associated with this property of special stability of aromatic systems results from the delocalization of electrons.

Aromatic Definition

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The classical definition of aromaticity was based on the cyclic nature the stability and the chemical reactivity of the compound, it was based on the aromatic character. Benzene is the aromatic compound but relatively few compounds possess benzene like properties.

Although aromaticity is not a function of the stability and chemical reactivity it is a function of the electronic structure and it is the main object of the present essay to define aromaticity with reference to the electronic structure of the molecule or ion.

"An unsaturated cyclic or polycyclic molecule or ion may be classified as aromatic if all the annular atoms participate in a conjugated system such that in the ground state all the $\pi$ electrons are accommodated in bonding molecular orbitals in a closed shell."

Aromatic Hydrocarbons

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Define Aromatic Hydrocarbons

Organic compounds are hydrocarbons or derivatives of hydrocarbons. Hydrocarbons contain only H and C atoms. The three mail groups of hydrocarbons are saturated hydrocarbons, hydrocarbons with only single bonds between the carbon atoms unsaturated hydrocarbons, hydrocarbons that contain double or triple bond between carbon atoms and aromatic hydrocarbons, hydrocarbons that contain a benzene ring.

Aromatic hydrocarbons are a class of polyenes which are distinctly different from common alkenes. They are generally classified as arenes. Most arenes are related to the six-member conjugated carbocycle benzene. Their chemical properties are associated with a unique type of conjugation known as aromaticity.

List of some of the aromatic hydrocarbons are given below.

Aromatic Hydrocarbons

Aromatic Amino Acids

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  1. The aromatic amino acids have been grouped together because they all contain ring structures with similar properties, but their polarity differs a great deal.
  2. The aromatic ring is a six member carbon hydrogen ring with three conjugated double bonds. These hydrogen atoms do not participate in hydrogen bonding.
  3. The substituents on this ring determine whether the amino acid side chain engages in polar or hydrophobic interactions.
  4. In amino acid phenylalanine the ring contains no substituents and the electron are shared equally between the carbon in the ring resulting in a very non polar hydrophobic structure in which the rings can stack on each other.
  5. In tyrosine a hydroxyl group on the phenyl ring engages in hydrogen bonds and the side chain is therefore more polar and more hydrophilic.
  6. The more complex ring structure in tryptophan is an indole ring with a nitrogen that can engage in hydrogen bonds. Tryptophan is therefore also more polar than phenylalanine.

Aromatic Amines

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Aromatic amino compounds are of two kinds the amino compounds in which amino group is directly attached to aromatic nucleus and they are known as aryl amines or nuclear aromatic amines. When amino group is present in side chain they are called aryl alkyl amines. These differ from aryl amines and resemble with aliphatic alkyl amines.

Amines are further classified as aliphatic or aromatic. An aromatic amine is one in which one or more of the groups bonded to nitrogen are aryl groups. The structural formula for aromatic amines is Ar-NH2, where Ar represents a homo aromatic or hetero aromatic ring. Aromatic amines are the most chemically reduced members of a larger class of N-aryl compounds.

A common example for aromatic amine s aniline. Some of the structures of aromatic amines are given below.

Aromatic Amines

Aromatic Rings

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Aromatic rings are commonly found in most drugs. Carbocyclic six membered aromatic rings are widely present in both natural products and endogenous ligands, and are therefore commonly incorporated by medicinal chemists into new chemical entities.

Aromatic rings are also preferred replacements of linear and branched alkyl/cyclo alkyl groups. Such a replacement not only imparts greater rigidity to the molecule but can potentially increase the non-covalent interactions between the macro molecule and the small molecules. These interactions generally involve aromatic amino acid side chains of the receptor and aromatic and hetero aromatic rings of the ligand.

The presence of aromatic ring in a molecule also provides a platform for functionalisation of a compound. It serves as a core that helps to orient other groups in a structure in the right direction and therefore enhance the interactions with functionalists in the receptor.

Aromatic Substitution

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In an electrophilic aromatic substitution, an electrophile reacts with an aromatic ring to give overall replacement of a hydrogen atom with another substituent. In nitration for example, the nitronium ion (+NO2) replaces a proton producing nitro benzene.

Aromatic Substitution

Electrophilic substitution occurs in two steps, first the addition of the electrophile second the loss of a proton. The first step is generally rate determining. Therefore significant insight into the overall reaction process can be gained by considering the structure of the intermediate cation formed upon addition of an electrophile to benzene.

Aromatic Ester

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Aromatic esters usually burn with a smoky flame possess reasonably high boiling points and are sometimes crystalline solids. Phenyl esters usually give phenol upon distillation with soda lime. Esters of carboxylic acids derived from phenols are more difficult to hydrolyze and exchange, hence any alcoholic solvent can be used freely.

The experimental details already given for the detection and characterization of aliphatic esters apply equally to aromatic esters. Aromatic esters undergo reduction on the benzene ring but ethyl benzoate can be reduced electrochemically in ethanol.

Aromatic Compounds

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An aromatic compound has a molecular structure containing cyclic clouds of delocalized $\pi$ electrons above and below the plane of the molecule and the $\pi$ clouds contain a total of (4n+2)$\pi$ electrons. This is known as Huckels rule.

Aromatic Compounds

If n=1 we have 4 $\times$ 1 + 2 = 6 which means that any compound containing a total number number of six $\pi$ electrons is an aromatic compound. In the above structure of benzene that are three double bonds and six $\pi$ electrons and it is a planar molecule. Thus benzene follows Huckels rule and is an aromatic compound.
Fused Aromatic Compounds

Aromatic Analgesics

The aromatic analgesics are
  1. Paracetamol
  2. Phenacetin
  3. Acetanilid

Naming Aromatic Compounds

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Aromatic substance more than any other class of organic compounds have acquired a large number of common names. Although the use of such names is discouraged IUPAC rules allow for some compounds that is to be retained. Thus methyl benzene is commonly known as toluene, hydroxy benzene as phenol and amino benzene as aniline and so on.

Mono substituted benzene are systematically named in the same manner as other hydrocarbons with benzene used as the parent name. Thus C6H5Br is bromo benzene and C6H5CH2CH3 is ethyl benzene. The name phenyl is used for the -C6H5 unit when the benzene ring is considered as a substituent and the name benzyl is used for the C6H5CH2- group.

The common names of some aromatic compounds are given below.

S.No Name of the compound
Structure
1 Toluene
Toluene
2
Phenol
Phenol
3
Aniline
Aniline
4
Acetophenone
Acetophenone
5
Benzaldehyde
Benzaldehyde
6
Benzoic acid
Benzoic Acid
7 ortho-Xylene
Ortho Xylene
8 Styrene Styrene

Aromaticity Rules

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According to Robinson sextet theory a completely conjugated cyclic system containing 6$\pi$ electrons is aromatic. The aromaticity rules was put forward by Huckels. These rules were proposed by a German physicist Eric Huckel in 1931.

The main points are
  1. The molecule or ion must be flat or nearly so.
  2. Molecule must have cyclic cloud of delocalized $\pi$-electrons above and below the plane of molecule.
  3. $\pi$-electron cloud envelop all the carbon atoms of the cyclic system.
  4. The total number of $\pi$-electrons in an aromatic molecule must be (4n+2), where n is an integer that is n = 0, 1, 2 ...etc. According to Huckels rules the number of $\pi$-electrons will be 2 when (n=0), 6 when (n=1), 10 when (n=2), 14 when (n=3) and 18 if (n=4) etc.

Polycyclic Aromatic Compounds

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This is a class of organic compounds with a fused ring aromatic structure. polycyclic aromatic hydrocarbons results from incomplete combustion of organic carbon. The simplest form of polycyclic aromatic compound is naphthalene.

Polycyclic Aromatic Compounds